Propylene oxide (PO) is a valuable chemical that is used to make propylene glycol, propylene glycol ethers, 1,4-butanediol, and polyols for use in the production of polyurethane materials. Generally, PO is formed by reacting propylene with an oxidizing agent in the presence of a catalyst. PO is commercially produced by reacting propylene with an organic hydroperoxide, such as ethyl benzene hydroperoxide, cumene hydroperoxide or tert-butyl hydroperoxide, in the presence of a solubilized molybdenum catalyst or a heterogeneous titania-on-silica catalyst. PO is also produced by the reaction of propylene and hydrogen peroxide in the presence of a titanium silicate catalyst.
In these processes, small amounts of hydrocarbons and oxygen-containing byproducts, such as methanol, acetone, methyl formate, and aldehydes, are produced. Many methods have been developed to remove these impurities from PO. Previous disclosed methods include extractive distillation techniques which utilize: C8 to C20 alkanes, alkenes or naphthenes, C6 to C12 aromatic hydrocarbons, C8 to C12 aliphatic or cyclic paraffins, and a tertiary butyl alcohol-water mixture to remove contaminating hydrocarbons (see U.S. Pat. Nos. 3,843,488, 3,909,366, 3,464,897, 5,006,206); and water or lower glycols such as ethylene glycol and propylene glycol to remove oxygen-containing impurities (see U.S. Pat. Nos. 4,140,588, 3,578,568 and 5,000,825).
Other purification processes include methods to remove methyl formate by contacting crude PO with alkali metal hydroxides, including: an aqueous alkali metal hydroxide solution (see U.S. Pat. No. 2,622,060); an aqueous solution of an alkaline saponifying agent (see U.S. Pat. No. 2,550,847); an aqueous slurry of calcium hydroxide (see U.S. Pat. No. 3,477,919); and sodium hydroxide in water and glycerol (see U.S. Pat. No. 4,691,035). Other methods include using a combination of distillation and a caustic treatment to simultaneously aldolize acetaldehyde and saponify methyl formate (see U.S. Pat. No. 3,350,417) and treating with an aqueous calcium hydroxide slurry to which a solubilizer and an aldehyde scavenger are added (see U.S. Pat. No. 4,691,034).
Adsorption techniques have also been taught to remove high levels of impurities, including the removal of high molecular weight ethers from PO by treatment with an absorbent such as activated carbon (see U.S. Pat. No. 4,692,535) and the removal of methyl formate from contaminated PO by contacting with a basic ion exchange resin (see U.S. Pat. Nos. 5,107,002 and 5,106,458).
Commercially useful techniques include plural stage distillation processes to purify PO. See, for example, U.S. Pat. No. 3,881,996. This patent discloses distilling crude, propylene-free PO to remove acetaldehyde as an overhead product, then distilling the bottoms stream to separate PO as an overhead product from propionaldehyde and other higher boiling materials. This method can produce PO having very low levels of aldehyde (less than 10 ppm). However, distillation processes are extremely energy-intensive and there is a significant energy input required to achieve such low levels of aldehyde.
For most applications, it is important to reduce the amount of aldehyde impurities to about 30-50 ppm, or lower. However, for a minor number of applications, it is necessary to reduce the aldehyde impurity level to 10 ppm or lower.
In sum, new methods for the purification of propylene oxide are needed. We have discovered an effective, convenient method to purify propylene oxide.